Storm clouds coming for the Toyota Prius? The automaker acknowledges the hybrid may have braking problems.

The 2010 Toyota Prius, arguably the most important car in the Japanese maker’s line-up, is now under investigation for Unintended Brake Release, following mounting owner reports that the vehicle’s brakes might release after hitting a pothole or bump.

Toyota’s announcement comes more than a month after TheDetroitBureau.com’s Editor Ken Zino first reported on the Prius braking problem, citing complaints filed with the National Highway Traffic Safety Administration, as well as information provided by owners to this publication.

It appears the investigation may have been spurred by the Japanese Ministry of Transportation, however, which has been worried by complaints received about Prius which is, in Japan, now the country’s best-selling automobile.

For its part, Toyota still downplayed the problem, suggesting that it is a manner of “inconsistent brake feel,” rather than the actual release of brakes, as numerous owners have described to TheDetroitBureau.com.

"Braking" News!

“Toyota is currently in the process of confirming these reports and investigating the vehicle driving conditions under which the reported phenomenon occurs. It would be premature to comment until the investigation has been completed.”

Having a safety-related problem with Toyota is a serious matter, warn many analysts, beyond what such an issue might mean if it were to involve a Camry or Tundra. The Prius is the brand’s halo car, and it adds far more to Toyota’s image than the sales numbers alone would suggest, noted Dave Sargent, chief of automotive research for J.D. Power and Associates.

“It would be huge,” said Sargent, and it would further drive home to consumers the idea that Toyota’s current problems with safety are “less of a one-time problem and more like something systemic.”

Exactly what might be wrong with Prius brakes – if, indeed, there proves to be a problem? Curiously, there appears to be some differences in the reports filed by Japanese and American owners. In Japan, the Ministry of Transportation says complaints center around an “unusual” or inconsistent feel when a motorist tries to stop on slick surfaces. American motorists are reporting that Prius brakes can release entirely when the suspension is jarred.

It helps to understand that the hybrid’s brakes are a compound system; under light to moderate deceleration they “regenerate” energy that can be stored in the vehicle’s batteries. When more aggressive braking is required, a conventional hydraulic system takes over.

A senior source at Toyota told TheDetroitBureau.com that whatever the “issue” is, it very well may involve the Prius’s ABS, or anti-lock brake system, which is designed to prevent skids by rapidly pulsing the brakes on slick surfaces. That would make particular sense in terms of what Japanese owners describe, though it would be a little less clear why American Prius owners describe a different sort of situation, with full brake release. But either way, if the complaints are accurate, they could point to various sensors, mechanical devices or the brake system’s computer controller.

The latter is particularly worrisome as Toyota has repeatedly ruled out the possibility of electronic problems with its vehicles, at least as related to the two currently active recalls for “unintended acceleration.” In October, the maker blamed “carpet entrapment” for a problem with 3.8 million vehicles – a number that has grown to nearly 5.5 million vehicles since then. Meanwhile, another recall of vehicles with potentially sticky accelerators has now spread around the world.

But as recently as Monday, Jim Lentz, president of Toyota Motor Sales USA, insisted the company had found all potential causes of unintended acceleration, concluding, “In terms of electronics, we’ve been extensively testing and found no evidence of problems.”

While Toyota is now studying the reported issues with Prius, the company is coming under increasing fire from a number of quarters. On Tuesday, U.S. Secretary of Transportation Ray LaHood declared the automaker, “safety deaf,” and declared the acceleration problems “the most serious safety issue” his department has faced.

Meanwhile, reports continue to come in about other problems with Toyota vehicles. As TheDetroitBureau.com reported yesterday, Steve Wozniak, a co-founder of Apple and a personal computer pioneer, is reporting that an apparent cruise control glitch can cause his 2010 Prius to “go wild.” Wozniak said that, as of yesterday, he has been unable to reach senior Toyota officials to outline for them a problem he said he can reaily recreate – a rarity, as most electronic glitches appear to be intermittent, at most.

Toyota’s problems are beginning to take a toll on the company. Its already weakened stock dove more than 5% during the most recent trading on Japan’s Nikkei, and in the critical U.S. market, sales plunged 16% for January, even as Detroit automakers reported a collective double-digit gain. General Motors and Ford, along with a number of import brands, are now targeting Toyota owners with aggressive incentive programs.

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I don’t think this is just with the 2010 Prius…I drive a 2006 model here and have has this issue since I purchased it in June 2006 or so. I’ve been told by dealers “thats just how the ABS is, don’t pump the brakes just apply firm steady pressuee”.

Hi, George,
Thanks for the comment and glad to have you join as a new subscriber!
It’s interesting to hear you mention a problem with the ’06 model’s brakes. The question is whether what you experience is the same as what the owners of the Gen-3 2010 Prius are noticing, which is that when they specifically hit something that jars the suspension the brakes appear to fully release? And yes, pumping the brakes is generally a no-no with ABS. The distinction might be if the brakes simply stop working and you need to release and reapply them, as some owners have said, just to get them working again.
Paul A. Eisenstein
Publisher, TheDetroitBureau.com

The experience is pretty much what is getting reported. I hit a bump, pothole or bumpy road surface, apply the brakes at the time and they seem to either not catch at all or catch and release. Its like a feeling of no braking action at all. Zero. Releasing the pedal and reapplying the brakes seems to fix the issue.

One difference in the Japanese vs. American reported experiences might be the difference in the road surfaces. Japanese drivers may not experience the suspension jarring – release event very much because they just don’t get nearly as much suspension jarring to begin with. I haven’t been to Japan in about eight years, but the last time I was there I noticed that all of their public infrastructure, roads included, was in much better repair than here. I don’t remember seeing the kind of irregular pavement, potholes, bumps, etc there that we experience on an everyday basis here. Now, I’m sure Toyota tests their vehicles on roads in all sorts of conditions, but their testing seems to have missed a few things of late.

Probably true, but ita not like we”re goinf off-road with the vehicles either. These are suburban streets in a major mtero area (Milwaukee WI) and in decent reapir for the most part.
If the car is that picky in braking there is a serious problem. Noone drives on smooth as glass roads in real life.

I found a certain bump that allows me to recreate this braking issue at will with my 2010 Prius, and I’ve tested the condition many times. The regenerative braking system in the 2010 Prius is remarkably effective and produces significant stopping forces. When either of the front wheels loose traction, even for a fraction of a second, the regenerative braking system is turned off. The reason for this probably has to do with the regen system not being able to pulse the brakes the way an ABS system can do, and if left on might cause further skidding when traction resumes. The impulse loads caused by this might also stress other components in the hybrid drive system.

From my experience, when this happens the conventional, all-wheel, friction brake system takes over within a half-second or so. The “feeling” that the driver experiences when this happens is that of acceleration, since the rate of slowing has been decreased. I don’t believe the car actually accelerates, but the feeling is rather unsettling even if you’re aware of the possibility of it happening.

I’m certainly no engineer, but I’d think that the solution to this issue would be to kick the friction brakes in a little faster and with greater force after a regen shutdown. I wonder if this fix might just involve downloading updated programming code to the on-board computer systems?

Hi, Lewie,
Wow, thanks for what has been up to now, the most detailed examination of what happens with the Prius brakes. You’re also the first person who has mentioned specifically being able to reproduce the issue, though other reader/owners have mentioned having the problem reoccur at certain locations but not necessarily every time.
Can you go into a bit more detail, ie speed at impact, whether you were light or heavy on the brakes at the time, etc.?
Paul A. Eisenstein
Publisher, TheDetroitBureau.com

Another bad break (no pun intended) for them. I really like their video that was released by the COO on the previous recall. At least they publicly acknowledged the mistake and made it a priority to fix the issue.